Hello to all, i am new to this site

I own a 2003 2.5S altima , recently i have noticed that my fuel Consumption has gone up the roof
first i have to let you know that i am from canada so my numbers could differ to yours
so here it goes with a full thank off premium gas i am getting 420 km (260 miles)
what could cause such a drop have cheked the tires and they are fine
i love my altima but she is making me worry
before i go to the dealer and he tells me his story could you guys let me know what could cause
such a drop.
thanks a much.